refactor(server/coverart): provider interface takes ArtistRef/AlbumRef

Changes the AlbumCoverProvider and ArtistArtProvider interfaces from
MBID-only string parameters to ref-struct signatures so name-based
providers (Deezer, Last.fm in upcoming tasks) can act on rows
without MBIDs.

Today, 53 of 213 artists in the dev DB have NULL MBIDs — featured
artists, remixers, and compilation contributors created from
track-tag splits where the primary-artist MBID is in the file but
collaborator IDs aren't. These rows are unreachable by the current
MBID-only chain. Refactoring the interface unblocks future
name-based providers from acting on them.

TheAudioDB and MBCAA keep MBID-only behavior internally: they return
ErrNotFound when ref.MBID is empty rather than attempting a
name-based fallback. The MBID guard inside EnrichAlbum/EnrichArtist
is removed; providers receive the ref unconditionally and decide
whether they can act on it.

GetAlbumWithFirstTrackPath now JOINs artists to return artist_name
alongside the existing fields, supporting AlbumRef construction.

No behavioral change today (only TheAudioDB + MBCAA registered, both
keep MBID-only behavior). The change unblocks T3 (Deezer) and T4
(Last.fm) which can now register and act on every artist/album
regardless of MBID presence.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/coverart/enricher_test.go

@@ -532,8 +532,8 @@ func (p *testAlbumProvider) DisplayName() string              { return p.id }
 func (p *testAlbumProvider) RequiresAPIKey() bool             { return false }
 func (p *testAlbumProvider) DefaultEnabled() bool             { return true }
 func (p *testAlbumProvider) Configure(ProviderSettings) error { return nil }
-func (p *testAlbumProvider) FetchAlbumCover(ctx context.Context, mbid string) ([]byte, error) {
-	req, err := http.NewRequestWithContext(ctx, http.MethodGet, p.baseURL+"/"+mbid, nil)
+func (p *testAlbumProvider) FetchAlbumCover(ctx context.Context, ref AlbumRef) ([]byte, error) {
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, p.baseURL+"/"+ref.MBID, nil)
 	if err != nil {
 		return nil, err
 	}